Methods have been developed for enrichment and characterization for human natural killer (NK) cells. The morphology, functional characteristics and certain surface characteristics of human NK cells have been described. The effect of interferon on the fractionated human NK cells has been studied. We have demonstrated that NK cells produce interferon when contacted with tumor target cells and the produced interferon recruits an initially non-cytoxic "pre-NK" cell population to killer cells with the properties of NK cells. There is thus, within the NK system itself a self-amplifying positive feed-back mechanism which may be potentially important as a first line of defence against tumor cells. Using the morphology of the human NK cells as markers we have improved the fractionation methods and obtain 85-90 percent pure fractions of such cells for continuation studies. The presence of NK cells within human solid tumors has been investigated and the program on the specific immunological characteristics of normal and neoplastic glial cells continued. Various other tumor markers have been investigated and also applied to clinical diagnostic work.